CSIRO PUBLISHING www.publish.csiro.au/journals/fpb Functional Plant Biology, 2009, 36, 960–969
The shoot and root growth of Brachypodium and its potential as a model for wheat and other cereal crops
Michelle Watt A,C, Katharina Schneebeli A, Pan Dong A,B and Iain W. Wilson A
ACSIRO Plant Industry, GPO Box 1600, Canberra, ACT 2601, Australia. BTriticeae Research Institute, Sichuan Agricultural University, Yaan, Sichuan 625014, China. CCorresponding author. Email: [email protected]
Abstract. The grass genetic model Brachypodium (Brachypodium distachyon (L.) Beauv., sequenced line Bd 21) was studied from germination to seed production to assess its potential as a phenotypic model for wheat (Triticum aestivum L.) and other cereal crops. Brachypodium and wheat shoot and root development and anatomy were highly similar. Main stem leaves and tillers (side shoots) emerged at the same time in both grasses in four temperature and light environments. Both developed primary and nodal axile roots at similar leaf stages with the same number and arrangement of vascular xylem tracheary elements (XTEs). Brachypodium, unlike wheat, had an elongated a mesocotyl above the seed and developed only one fine primary axile root from the base of the embryo, while wheat generally has three to five. Roots of both grasses could develop first, second and third order branches that emerged from phloem poles. Both developed up to two nodal axile roots from the coleoptile node at leaf 3, more than eight nodal axile roots from stem nodes after leaf 4, and most (97%) of the deepest roots at flowering were branches. In long days Brachypodium flowered 30 days after emergence, and root systems ceased descent 42 cm from the soil surface, such that mature roots can be studied readily in much smaller soil volumes than wheat. Brachypodium has the overwhelming advantage of a small size, fast life cycle and small genome, and is an excellent model to study cereal root system genetics and function, as well as genes for resource partitioning in whole plants.
Additional keywords: Arabidopsis, architecture, genome, monocotyledons, photoperiod, root anatomy, tillering.
Introduction growth are similar to Brachypodium, their root–rhizosphere This paper presents a description of Brachypodium distachyon interactions may be similar, as well as root architectures for (L.) Beauv. (Brachypodium) sequenced line Bd 21 shoot and root different moisture and nutrient availabilities. Importantly, at systems from the seedling to grain filling stages. The aim was to maturity Brachypodium is much smaller than cereal crops, assess its potential to identify genes associated with the including other genetic models such as rice (Oryza sativa L.) development and function of wheat (Triticum aestivum L.) and maize (Zea mays L.) and it can be grown in controlled root systems. Brachypodium has been developed as a model conditions in small volumes of soil without greatly crowding for the genetics of cool climate grasses for fuel and crop cereals roots and compromising their growth and function (Fig. 1). (Draper et al. 2001; Opanowicz et al. 2008; Vogel and Bragg Brachypodium presents a new opportunity to identify genes 2009). It has all the desirable features of a genetic model including associated with the growth and function of the roots of a small genome, short generation time between emergence and important cereal crops such as wheat, especially at more seed yield, and easy growth requirements. It is an annual, mature plant stages that are very important in field grown cereals. inbreeding and can be readily transformed (Vogel et al. 2006). Arabidopsis thaliana (L.) is a small-sized genetic model, and The entire genome of diploid line Bd 21 is sequenced and has been the source of most genetic information for roots for publically available (J. Vogel, unpubl. data; http://www. nearly 20 years (Schiefelbein and Benfey 1991; Osmont et al. Brachypodium.org/, accessed 30 September 2009). 2007). However, it is a dicotyledon and member of the Brachypodium originates from the Fertile Crescent, the Brassicaceae family. Dicotyledons develop a tap root system geographic origin of wheat, and is considered a temperate with vascular cambia that produce new xylem and phloem with grass suited to cooler conditions (Vogel et al. 2009). On the development (Esau 1977). Monocotyledons like Brachypodium, phylogeny trees of the grass family Poaceae, the genus and all members of the grass family, develop a ‘fibrous’ root Brachypodium is positioned with the temperate cereal genera system with no vascular cambia, and roots that emerge from the Triticum, Hordeum and Avena in the subfamily Pooideae base of the embryo within the seed (primary or seminal axile (Kellogg 2001; Opanowicz et al. 2008). It can be expected roots), and from successive nodes along the stem (nodal axile that the environmental and soil conditions favourable to wheat roots) (Klepper et al. 1984; Wenzel et al. 1989; Hochholdinger